Scientists convince people they've got three arms

One of the most irritating things about you Earthlings is that you’re never happy with what you’ve got. We gave you two arms for a bloody good reason – and sometimes, quite frankly, it seems like too many.

Grafting on extra limbs is a very messy business, and most budding Frankensteins have tended to focus on other parts of the anatomy. We’ve got a couple of confiscated prototypes in the post-room, and, let me tell you, they make for a good way of passing the afternoon on a slow news day.

But scientists at Sweden’s Karolinska Institutet wanted research results that they could publish in a family scientific journal – PLoS One, since you ask. So they played it a bit safe, and successfully persuaded a bunch of test subjects that they had three arms.

They sat them at a table with a realistic prosthetic right arm next to their own. Then, in a bit of head-messing that you really have to admire, they touched the subjects’ right hands and rubber hands with two small brushes in the same spot, synchronising the strokes as perfectly as possible.

“What happens then is that a conflict arises in the brain concerning which of the right hands belongs to the participant’s body. What one could expect is that only one of the hands is experienced as one’s own – presumably the real arm,” says researcher Arvid Guterstam.

“But what we found, surprisingly, is that the brain solves this conflict by accepting both right hands as part of the body image, and the subjects experience having an extra third arm.”

To make sure their victims weren’t just being polite, the team threatened them with a kitchen knife, holding it near either the prosthetic hand or the real hand.

Rather than calling the police, the subjects simply dripped with sweat – they’re a bit timid in Sweden, you see. They showed exactly the same stress response when the prosthetic hand was threatened as when the real hand was. This only happened while they were experiencing the third arm illusion, and not, for instance, when the prosthetic right arm was replaced with a left arm or a prosthetic foot.

You’d have thought that Swedes would be really quite imaginative when it comes to putting a third arm to use. But this lot must be born-again Christians or something.

“It may be possible in the future to offer a stroke patient, who has become paralysed on one side of the body, a prosthetic arm that can be used and experienced as his own, while the paralysed arm remains within the patient’s body image,” says study leader Henrik Ehrsson.

“It is also conceivable that people with demanding work situations could benefit of an extra arm, such as firemen during rescue operations, or paramedics in the field.”